The technical development and demand for mobile devices is increasing. Since secondary batteries have the properties of being rechargeable, capable of being manufactured in small sizes, and having high-capacity, there is a rapidly increasing demand for these batteries as an energy source for electrical and electronic devices including mobile devices, and many studies are being conducted on them.
Particularly, in terms of the shapes of the secondary batteries, prismatic batteries capable of being applied to products such as mobile phones due to their thin thicknesses and pouch-type batteries are in high demand. In terms of the materials for secondary batteries, the demand is increasing for lithium secondary batteries such as lithium cobalt polymer batteries having superior energy density, discharge voltage, and safety.
In general, lithium secondary batteries may be exploded due to high temperatures and pressures inside the batteries that are caused by abnormal operation states such as an internal short circuit, an overcharge, a state where the battery is exposed to high temperature, a falling impact, and so on. In such a secondary battery, one of the main subjects being studied is the improvement of battery safety.
In a process of manufacturing a pouch-type lithium ion polymer battery 101, there is a need to measure the thickness of a polymer battery packing material 103 and a polymer battery packing material sealing part 105 to secure safety (see FIG. 2, for a configuration of the lithium ion polymer battery). When the polymer battery packaging material 103 has a thickness over an allowed range, this leads to a defect in product and a reduction in uniformity, and ultimately, a poorly assembled product. Furthermore, there is the risk of battery explosion that may occur in the above-described abnormal operation states of the battery.
When the polymer battery packaging material sealing part 105 (hereinafter, referred to as a “sealing part”) has a thickness over the allowed range, this is a defect of the sealing part 105. When the sealing part 105 has a defect, moisture in the air may be easily introduced into the battery to cause deterioration in the performance of the battery and in the long-term stability of the battery, for example, battery corrosion. Therefore, it is necessary to measure the thicknesses of the polymer battery packaging material 103 and the sealing part 105 to separate defective products and to examine the rate of defects.
The thicknesses of the polymer battery packaging material 103 and the sealing part 105 have been manually measured by workers using a contact micrometer. However, such a contact micrometer is inefficient, because the measurement speed is slow, and reproducibility of the measurement is reduced when the contact micrometer is used. Also, when the thicknesses are measured by using the contact micrometer, the contact trace may remain on the measured object, so that the measurement lacks accuracy.
Moreover, to identify the thickness distribution of the polymer battery packaging material 103 and the sealing part 105, many measurement points need to be measured, and there is thus a limitation in that the thickness distribution on a specific line or plane of the polymer battery packaging material 103 and the sealing part 105 may not be identified.